The present invention relates to magnetic tape drive systems, and more particularly, to a non-slip tachometer roller for use in a magnetic tape drive assembly.
Magnetic storage disks, commonly referred to as hard disks, are presently a preferred storage medium for computer systems where short access time and good storage capacity are of interest. But magnetic storage tapes are commonly used because of their low cost, portability and adequate storage capacity. Contrary to hard disks, which generally remain inaccessible within the disk drive assembly, the tape in its removable container (called a cartridge) can be more economically transported to remote locations.
A typical magnetic tape includes a mylar substrate, having a coating of magnetic recording material on one side and a non-stick "back coating" on the other side of the mylar. The magnetic recording material is sensitive to magnetic fields generated by a magnetic read/write head located in the computer tape drive assembly. The magnetic fields from the head represent information to be stored as data in tracks on the tape, and cause changes (i.e., transitions) in the magnetic (i.e., dipole) orientation of the magnetic material on the tape. The head in turn reads the fields of the stored transitions in a given track on the tape and generates a readback signal representative of these transitions for later decoding in order to retrieve stored data.
In order to increase storage density for a given cartridge size, thinner tape may be employed so as to put more tape on the supply reel. Also, data may be written on the tape in any one of a plurality of parallel longitudinal tracks as the tape streams by the head along a longitudinal tape path. If the data portion of the tape puckers, is warped or otherwise is not essentially planar as it contacts the sensing portion of the head, then data retrieval errors are likely to result. These errors are commonly referred to as "drop-outs", since the data on the deformed part of the tape drops out of the retrieved data.
The tape runs between a supply reel in the tape cartridge and a takeup reel in the computer's tape drive assembly, and must be maintained at a constant speed as it streams across the magnetic head during reading or writing to avoid data errors. A drive motor typically is associated with each reel, and is driven under control of a motor controller circuit which receives information from a tachometer coupled to a roller, with the roller placed in the tape travel path. The tape streaming across the roller surface rotates the roller which motion is sensed by the tachometer. The tachometer then generates a control signal assumed to be representative of tape speed, and the control signal is employed by the motor controller circuit to regulate the speed of the drive motors and hence tape speed. In this manner tape speed is servoed at or about a desired operating point.
However, if the tape slips on the tachometer roller, then an incorrect (low) tape speed signal will be generated by the tachometer, because the tape will be moving faster than the roller. This slippage error in the tape speed signal, i.e., the disparity between tape and roller speed, is problematic. The motor controller will accelerate the drive motors in response to the incorrectly low tape speed signal, and so the tape will be accelerated. This acceleration will further increase the slippage error between tape and roller since the roller is even less likely to capture the tape at higher tape speed. Hence this slippage error continues to increase each time the motor controller updates its motor drive output based on the erroneous tape speed signal. This leads to a "runaway" condition and will cause a system shutdown. Tape slippage can also cause difficulty in location of data on the tape.
Tape slippage in part is attributed to a thin film of air which flows between the streaming tape and the roller. The air film lifts the tape off of the roller, sufficiently to cause an incorrect tape speed indication as the tachometer roller fails to turn at the same rate as the tape is moving. It is therefore necessary to vent the air film between the tachometer roller and the streaming tape in order to reduce slippage between the roller and tape.
While having a plurality of radial grooves on the tachometer roller periphery might be employed for venting of the air film between the tape and roller, very thin tape tends to cave into the grooves, which deforms the tape. If these deformations occur along a data track, which is likely when using a roller with a multiplicity of radial grooves and a thin multi-track tape, then data dropouts are likely to occur. Therefore, it has been found that for particularly thin magnetic recording tape, such as tape having 0.0005 inch (i.e., half mil) thickness, it is not practical to employ a multiplicity of radial grooves on a tachometer roller for venting of the air film. Furthermore, the many tool plunges required to cut a multiplicity of radial grooves increases tooling expenses and raises quality control problems as burrs are often formed during the tool plunging process. These burrs will damage the tape and therefore must be removed by polishing during manufacture.
It is therefore an object of the present invention to provide a high density tape drive which provides an accurate indication of tape speed.
It is another object of the present invention to provide a non-slip tachometer roller for use in a high density tape drive.
It is a further object of the present invention to a provide tachometer roller for use with thin magnetic tapes, the roller having improved traction while maintaining tape integrity.
It is another object of the present invention to provide a high yield and cost-effective method for forming a non-slip tachometer roller for use with thin magnetic tapes.